The invention relates to a vacuum plasma generator for feeding a plasma discharge for the treatment of workpieces in a vacuum chamber.
Electrical feeder apparatus for operating gas discharges, or plasma discharges, for vacuum processes are known in a variety of manners. Such electrical feeder apparatus are technically also referred to as generators. It is here important to be able to control the operating conditions well and reliably, since the nature of plasma discharges and the plasma connected thereto, make special requirements of them. With such vacuum plasma processes today a multiplicity of different applications are covered, such as for example coating, cleaning, thermal treatment of workpieces and substrates, etc. Such plasma processes are, for example, cathode sputtering, spark evaporation, also referred to as arc evaporation, surface treatments through electron or ion bombardment and plasma CVD deposition methods. In order to be able to attain specific results in such plasma processes, the discharge processes must be mastered appropriately and controlled purposefully with the aid of suitable power supplies. The possible parameter field of the various operating modes of such plasma discharges is here very broad and it is possible to achieve novel and unexpected results, such as for example in the layer properties of coated workpieces. Attempts have therefore been made for some time to expand the parameter field additionally with plasma discharges fed pulsatingly in order to be able to realize new possibilities. Herein unipolar pulses or bipolar pulses are utilized at different frequencies, often in the range of kHz up to more than 100 kHz, and also with different pulse widths and/or pulse shapes. With this pulse technology it is possible to work, for example, with poorly conducting materials or even insulating layers without undesirable spark discharges occurring. In particular in the important application case of substrate bias, pulse technology leads to positive results. Here, as a rule, work took place with a voltage-stable pulsed current supply with which high currents can also be realized. For generating coatings it is also possible to feed using the pulse technique coating sources, for example sputtering sources or also spark evaporation sources.
Pulse technology is especially well suited for controlling processes, such as for example reactive processes, with materials that are poorly conducting or not at all. However, employing this technology also yields enormous advantages when using electrically well conducting materials in the process, such as for example for setting the stoichiometry of layers, the density of layer materials and also for exerting an influence on the structure.
Due to the diversity of feasibilities, work today takes place with the entire spectrum of known plasma discharge types or plasma operating modes.
In U.S. Pat. No. 5,303,139 a bipolar power supply is utilized, which describes extensive pulse plasma applications in the area of so-called glow discharges in the field of PVD and CVD applications. The glow discharges conventionally operate at voltages below 1000 V, thus at a few 100 V, and low current values. Such glow discharges are often additionally supported with specific magnet fields. Sources of this type have become known as magnetron sources. They are operated at a few 100 V and permit realizing higher discharge currents than the glow discharge without magnetic field support.
Abnormal glow discharges, such as are described for example in U.S. Pat. No. 5,015,493, are operated at higher voltages of a few kV and at higher current values than those of glow or magnetron discharges, however at lower current values compared to spark discharges.
Spark evaporators, also referred to as arc evaporators, are operated in the range of a few 10 V, however at very high currents of conventionally a few 100 A.
A further specific discharge form is disclosed in U.S. Pat. No. 6,296,742, the so-called “High Power Pulsed Sputtering”.
This operating mode is sometimes also referred to as “diffused discharge”, since a striking diffuse brightening of the plasma occurs in the target proximity. The discharge is herein operated with pulse voltages of 0.35 to 5 kV at pulse powers of 10 kW to 1 MW. The pulse lengths are in the range of 50 microseconds to 1 millisecond and the interval between the pulses is in the range from 10 milliseconds to 1000 seconds.
Said plasma processes or plasma treatment methods represent a load for the power supply, which cover a broad impedance range and, consequently, require feed voltages in the range of a few V up to the kV range and coil currents of a few A to a few 100 A at pulse frequencies from kHz to a few 100 kHz. Until now for each application range separate power supplies had to be utilized laid out according to the load or to the corresponding plasma operating mode. If a further voltage range is to be covered with conventional generators, the disadvantage is encountered that the nominal power of the generator must correspond to the product of maximal output voltage and maximal output current, and methods which require low voltages most often require higher currents than methods with high voltage, such that the nominal power would need to become unnecessarily large.
A further disadvantage of conventional generators employed over a wide output voltage range is the relative lowering of the quality when operating with low output voltage. For example, thereby the resolution is decreased at a simultaneous increase of the ripple. A further especially serious disadvantage comprises that the pulse form is often not stable and varies during the operating as a function of the load behavior and, in this case, especially the edge steepness of the pulses varies or even flattens and sufficiently steep pulses edges cannot be generated. With the plasma processes currently carried out for working workpiece surfaces, such as in particular for the deposition of layers, it should be possible to operate several of the plasma operating modes previously listed within the production process in the same vacuum process installation. This should be possible in individual plasma source configurations through the control of the operating mode and/or also in several different source configurations, which are specifically laid out for different operating modes, such as for example, and preferably, a combination of arc evaporators, sputter sources and bias plasma gaps.